This invention relates to a method for carrying out a color correction in a digital manner in a picture reproducing machine such as a color scanner, color facsimile, or the like, which is capable of saving a large volume of the capacity of a memory used therein.
In a color printing, when color separation printing plates are made, color separation signals obtained by scanning an original picture are generally color-corrected in order to remove inconveniences according to the hue differences among color printing inks and the gradation differences of halftone dots produced by means of halftone screens, and the like.
For instance, in a conventional picture reproducing machine such as a color scanner, or the like, the color correction is performed in an analog system, wherein the hue of the original picture is divided, for instance, into six color signals for violet, magenta, orange, yellow, green and cyan. Then, from the six color signals are obtained four color signals for cyan C, magenta M, yellow Y, and black Bk, which control the amounts of the color printing inks for each printing plate.
This method, however, has defects such as lack of stability of the processing circuits, limited color correction variable range, and so forth.
In order to remove these defects, a digital computer system having a memory is proposed, and it has advantages such as wide correction variable range, convenience of operation, simple construction, high quality printing and rapid operation. In this case, in general, an original color picture is scanned to obtain three (red, green, and blue) color separation signals R, G and B. These three color separation signals are sent to a color operation circuit, thereby finally obtaining recording signals for density of printing inks, such as cyan C, magenta M, yellow Y, and black Bk.
In order to provide the most accurate possible color reproduction, a combination of the amounts of cyan, magenta, and yellow inks (the black ink, and so forth, are omitted for the sake of brevity of explanation) is necessarily determined corresponding to a combination of red, green, and blue color separation signals.
Consequently, for the purpose of color correction by selecting the combination of cyan, magenta, and yellow values corresponding to the combination of red, green, and blue values, the color-corrected combinations of cyan, magenta, and yellow values corresponding to each combination of red, green, and blue values are stored in a memory in advance, and then, the color-corrected combination of cyan, magenta, and yellow values is read out by addressing the memory by the combination of red, green and blue values corresponding thereto.
If each red, green, and blue range is divided into, for example, two hundred tone steps, altogether 200.sup.3 =8,000,000 combinations of cyan, magenta, and yellow values must be stored in the memory, which requires that the memory has a large capacity. Therefore, in practice, in order to reduce the storage capacity required for the memory, the tone steps to be divided are reduced to a manageable level, and intermediate values are properly interpolated between each two tone steps in the three-dimensional space defined in the memory by the three axis of red, green and blue. However, this method still requires the memory having a large capacity. This means high cost, and thus is not practicable.